Objective: To analyze the value of the hCG ratio of peritoneal serum versus venous serum (RP/V) for early diagnosis and prognostic evaluation of ectopic pregnancy (EP). Design: Retrospective clinical study. Setting: University hospital. Patient(s): One hundred three women with hemoperitoneum and positive urine hCG tests underwent laparoscopy or laparotomy. Intervention(s): Venous serum and peritoneal serum samples were obtained; ultrasound was performed in all the patients; dilatation and curettage was used in 28 patients. Main Outcome Measure(s): Quantitative hCG and RP/V. Result(s): The RP/V in EP (5.55 ± 4.32) is apparently greater than that in hemoperitoneum with intrauterine pregnancy (hIUP; 0.61 ± 0.18). The median RP/V is 4.07 in the EP group versus 0.60 in the hIUP group, with a suggested threshold value of 1.0 for their differential diagnosis. Moreover, the RP/V of EP shows the dominant difference between the patients with active bleeding (8.03 ± 3.29, n = 24) and the patients without active bleeding (4.59 ± 3.88, n = 16) when the hCG level of venous serum is more than 1500 U/L. Conclusion(s): RP/V could instantly diagnose ectopic pregnancy and differentiate it from hIUP. © 2010 American Society for Reproductive Medicine.